Mask assembly



FIG. 1 is a front view of a mask assembly according to an embodiment of our new design;

FIG. 2 is a front view of the mask assembly thereof;

FIG. 3 is a rear view of the mask assembly thereof; the broken lines in FIG. 3 illustrate environmental structure of the mask assembly, which forms no part of the claimed design;

FIG. 4 is a top view of the mask assembly thereof; and,

FIG. 5 is a right side of the mask assembly thereof.

This design is directed to a mask assembly. Where included, broken lines illustrate environmental structure of the mask assembly, which forms no part of the claimed design. 

The ornamental design for a mask assembly, as shown and described. 